Broadly defined, electrical systems and devices are of the direct current (DC) or alternating current (AC) type. Of the two types, AC systems and devices are used predominantly if not almost exclusively to power residential, commercial and industrial electric motors, appliances, light fixtures and the like.
AC devices often use some sort of laminated structure to help reduce eddy current losses. For example, transformers have laminated cores and AC motors use laminated stators and rotors. But such loss reduction is not without its price -- the laminated device often exhibits what is referred to as "AC hum." In many situations, AC hum is either not audible or is barely audible to humans. This may be because the AC device is physically small and of low power. Or the AC hum may be masked by other sounds.
On the other hand, higher-power AC devices such as large transformers, ballasts for industrial lighting fixtures and similar devices exhibit an audible AC hum. For grounding and heat transfer purposes, ballasts with exposed laminated cores are often mounted with the ballast windings protruding parallel to a housing surface and a flat surface of the core abutted against the surrounding metal housing. The minute vibrations of the core laminations are thereby amplified by such housing. The sound level produced by a transformer or ballast alone may be relatively innocuous but when amplified, such vibrations may be (and often are) annoying to persons nearby.
An electrical apparatus with a feature suppressing noise resulting from AC hum would be a desirable advance in the art.